It has been widely known that, in a steelmaking process, at the time of supplying a molten steel primary-refined by a converter or an electric furnace to a casting process represented by continuous casting, the molten steel is treated in advance by a simplified secondary refining apparatus for the purpose of adjusting the chemical composition and temperature.
Japanese Unexamined Patent Publication (Kokai) No. 53-149826 discloses a process for heating a molten steel comprising immersing an immersion snorkel in a molten steel to provide a protective wall while the molten steel is being stirred by blowing a gas through a gas-blowing nozzle in the bottom of a ladle, and blowing oxygen gas onto the molten steel through an oxygen-blowing pipe while an oxidation reaction agent is being added to a region surrounded by the protective wall through a supply snorkel. Moreover, Japanese Unexamined Patent Publication (Kokai) No. 61-235506 discloses a process for refining a molten steel in a ladle comprising blowing an oxidizing gas through a lance onto the surface of a molten steel within an immersion snorkel inserted into a ladle, while the molten steel is stirred by blowing an inert gas through the bottom of the ladle, wherein prior to blowing the oxidizing gas through a top-blowing lance, an oxidation reaction agent is added within the immersion snorkel, and then blowing the oxidizing gas and adding the oxidation reaction agent are continuously conducted through the top-blowing lance.
However, although the structure of the top-blowing lance and the conditions for blowing oxygen significantly influence the efficiency for heating a molten steel, neither the influences nor the conditions for efficiently heating a molten steel are disclosed in the patent publication. Japanese Unexamined Patent Publication (Kokai) No. 61-129264 discloses a process for refining a molten steel in a ladle comprising inserting an immersion snorkel into a ladle while a molten steel is being stirred by blowing an inert gas through the bottom of the ladle, and blowing an oxidizing gas onto the molten steel surface within the immersion snorkel through a lance, wherein an annular lance comprising an inner pipe and an outer pipe is used as the lance mentioned above, oxygen and an inert gas are supplied to the inner pipe and the outer pipe, respectively, while the blowing pressure P1 of the oxidizing gas from the inner pipe is being made lower than the blowing pressure P2 of the inert gas from the outer pipe (P1&lt;P2), and an oxidation reaction agent is added within the immersion snorkel in accordance with the conditions for blowing oxygen.
However, the process has the problem of a high gas cost because a large amount of a costly inert gas is used therein. On the other hand, Sumitomo Kinzoku, vol. 45-3, page 66 and the following pages (1993) discloses an embodiment of using a castable annular pipe as an oxygen lance. However, since a lance having a castable structure lacks durability, the lance has the disadvantage of being costly.